Valve amplifier loosing 'gain' over 20 minutes
"Rudge" wrote in message
...
A friend has a 1950's Tannoy PA valve amplifier which works when first
turned on but the output steadily drops to zero after 20 minutes.
That's his description of the fault. I have not tested the amp yet.
Thinking about it, I'm assuming it is a problem with a capacitor.
I'm not familiar with all the failure modes of old capacitors. I know that
old electrolytics loose capacitance and become leaky when the dielectric
degrades.
Could this fault be caused by a leaky interstage capacitor which is
affecting the bias of the next valve stage?
Or could it be a valve problem?
Dried out electrolytics usually improve as they warm up, leaky capacitors
usually worsen with temperature which among other things can cause biasing
faults - glowing tube plates is a dead giveaway!
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